Percussion drilling is a well-established technique that breaks rock by hammering impacts transferred from the rock drill bit, mounted at one end of a drill string, to the rock at the bottom of the borehole. The energy needed to break the rock is generated by a hydraulically driven piston that contacts a shank adaptor positioned at the opposite end of the drill string to the drill tool. The piston strike on the adaptor creates a stress (or shock) wave that propagates through the drill string and ultimately to the borehole rock bottom.
Shank adaptors typically comprise an internal bore to allow transfer of a flushing fluid to the region of the drill tool. The flushing fluid acts to both cool the tool and to expel drill cuttings and fines from the bore hole. Conventionally, the fluid is introduced into the shank adaptor via a radially extending hole in the adaptor wall that is submerged within a fluid tank that seals onto the external surface of the adaptor axially either side of the hole. Example shank adaptors with internal flushing bores are described in EP 1077305; WO 2013/109182; WO 2004/079152 and U.S. Pat. No. 4,094,364.
A common problem with existing shank adaptors is the susceptibility for the adaptor wall to fracture due to compressive and tensile stresses generated by the percussive piston and bending moments due to lateral deviation of the drill string during drilling, with the fault originating and propagating from the flushing hole. Shank adaptor failure is typically sudden and results in downtime of the drilling assembly. Whilst WO 2004/079152 discloses a flushing hole intended to reduce failure of the adaptor, there still exists a need for an adaptor having a flushing hole that further reduces or eliminates the likelihood of fracture in response to both compressive and tensile forces and bending moments.